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DIAZO COMPOUNDS
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DIAZO COMPOUNDS , in organic See also:chemistry, compounds of the type R•N•2•X (where R = a See also:hydrocarbon See also:radical, and X = an See also:acid radical or a hydroxyl See also:group). These compounds may be divided into two classes, namely, the true diazo compounds, characterized by the grouping —N=N—, and the diazonium compounds, characterized by the grouping N:N<.
The diazonium compounds were first discovered by P. Griess (See also:Ann., 1858, 106, pp. 123 et seq.), and may be prepared by the See also:action of nitrous fumes on a well-cooled See also:solution of a See also:salt of a See also:primary amine,
C6H5NH2•HNO3 + HNO2 = C6H5N2.NO3 + 2H2O,
or, as is more usually the See also:case (since the diazonium salts
themselves are generally used only in aqueous solution) by the
addition of a well-cooled solution of See also:potassium or See also:sodium nitrite
to a well-cooled dilute acid solution of the primary amine. In
See also:order to isolate the anhydrous diazonium salts, the method of
E. Knoevenagel (Ber., 1890, 23, p. 2094) may be employed. In
this See also:process the amine salt is dissolved in See also:absolute See also:alcohol and
diazotized by the addition of amyl nitrite; a crystalline pre-
cipitate of the diazonium salt is formed on See also:standing, or on the
addition of a small quantity of See also:ether: The diazonium salts are
also formed by the action of See also:zinc-dust and acids on the nitrates
of primary See also:amines (R. Mohlau,Ber., 1883, 16, p. 3080), and by the action of See also:hydroxylamine on nitrosobenzenes. They are colourless crystalline solids which turn See also: They dissolve easily in See also:water, but only to a slight extent in alcohol and ether. They are very unstable, exploding violently when heated or rubbed. See also:Benzene diazonium nitrate, C6HsN(NO3):N, crystallizes in See also:long silky needles. The sulphate and chloride are similar, but they are not quite so unstable as the nitrate. The bromide may be prepared by the addition of See also:bromine to an ethereal solution of diazo-amino-benzene (tribromaniline remaining in solution). By the addition of potassium bromide and bromine water to diazonium salts they are converted into a perbromide, e.g. C6H5N2Br3, which crystallizes in yellow plates. The diazonium salts are characterized by their See also:great reactivity and consequently are important reagents in synthetical processes, since by their agency the amino group in a primary amine may be exchanged for other elements or radicals. The See also:chief reactions are as follows: 1. Replacement of—NI-I, by—OH:—The amine is diazotized and the aqueous solution of the diazonium salt is heated, See also:nitrogen being eliminated and a phenol formed. 2. Replacement of —NH2 by See also:halogens and by the — CN and CNO See also:groups:—The diazonium salt is warmed with an acid solution of the corresponding cuprous salt (T. Sandmeyer, See also:Bee., 1884, 17, p. 2650), or with See also:copper See also:powder (L. Gattermann, Ber., 1890, 23, p. 1218; 1892, 25, p. 1074). In the case of See also:iodine, the substitution is effected by adding a warm solution of potassium iodide to the diazonium solution, no copper or cuprous salt being necessary; whilst for the See also:production of nitriles a solution of potassium cuprous See also:cyanide is used. This reaction (the so-called Sandmeyer " reaction) has been investigated by A. Hantzsch and J. W. Blagden (Ber.,1900,33,p•254 ), who consider that three simultaneous reactions occur, namely, the formation of labile See also:double salts which decompose in such a See also:fashion that the radical attached to the copper See also:atom wanders to the aromatic See also:nucleus; a catalytic action, in which nitrogen is eliminated and the acid radical attaches itself to the aromatic nucleus; and finally, the formation of See also:azo compounds. 3. Replacement of — See also:NH2 by — NO2:—A well-cooled concentrated solution of potassium mercuric nitrate is added to a cooled solution of benzene diazonium nitrate, when the crystalline salt 2C6H5N2•NO3, Hg(NO2)2 is precipitated. On warming this with copper powder, it gives a quantitative yield of See also:nitrobenzene (A. Hantzsch, Ber., 1900, 33, p. 2551). 4. Replacement of —NH2 by See also:hydrogen:—This See also:exchange is brought about, in some cases, by boiling the diazonium salt with alcohol; but I. Remsen and his pupils (Amer. Chem. Journ., 1888, 9, pp. 389 et seq.) have shown that the See also:main product of this reaction is usually a phenolic ether. This reaction has also been investigated by A. Hantzsch and E. Jochem (Ber., 1901, 34, p. 3337), who arrived at the conclusion that the normal decomposition of diazonium salts by See also:alcohols results in the formation of phenolic See also:ethers, but that an increase in the molecular See also:weight of the alcohol, or the See also:accumulation of negative groups in the aromatic nucleus, diminishes the yield of the ether and increases the amount of the hydrocarbon formed. The replacement is more readily brought about by the use of sodium See also:stannite (P. Friedlander, Ber., 1889, 22, p. 587), or by the use of a concentrated solution of hypophosphorous acid (J. See also:Mai, Ber., 1902, 35, p. 162). A. Hantzsch (Ber., 1896,29,p. 947 ;1898, 31, p. 1253) has shown that the chlor- and brom- diazoniumthiocyanates, when dissolved in alcohol containing a trace of hydrochloric acid, become converted into the isomeric thiocyanbenzene diazonium chlorides and bromides. This See also:change only occurs when the halogen atom is in the ortho- or See also:para- position to the — N2— group. Metallic Diazo Derivatives.—Benzene diazonium chloride is decomposed by See also:silver See also:oxide in aqueous solution, with the formation of benzene diazonium hydroxide, C,H5.N(OH)i N. This hydroxide, although possessing powerful basic properties, is unstable in the presence of alkalis and neutralizes them, being converted first into the isomeric benzene-diazotic acid, the potassium salt of which is obtained when the diazonium chloride is added to an excess of See also:cold concentrated potash (A. Hantzsch and W. B. See also:Davidson, Bee., 1898, 31, p. 1612). Potassium benzene diazotate, CeH6N2.OK, crystallizes in colourless silky needles. The See also:free acid is not known ; by the addition of the potassium salt to 50% acetic acid at — 20° C., the acid anhydride, benzene diazo oxide, (CsH5N2)2O, is obtained as a very unstable, yellow, insoluble See also:compound, exploding spontaneously at o° C. Strong acids convert it into a diazonium salt, and potash converts it into the diazotate. On the constitution, of these anhydrides see E. See also:Bamberger, Bee., 1896, 29, p. 446, and A. Hantzsch, Ber., 1896, 29, p. 1067 ; 1898, 31, p. 636. By the addition of the diazonium salts to a hot concentrated solution of a See also:caustic See also:alkali, C. Schraube and C. See also:Schmidt(Ber., 1894, 27, p. 52o)obtained an isomer of potassium benzene diazotate. These iso-diazotates are formed much more readily when the aromatic nucleus in the diazonium salt contains negative radicals. Potassium benzene iso-diazotate resembles the normal salt, but is more See also:stable, and is more highly ionized. See also:Car-bon dioxide converts it into phenyl nitrosamine, C6H6NH•NO (A. Hantzsch). The potassium salt of the iso-diazo hydroxide yields on methylation a nitrogen ether, R•N(See also:CH3)•NO, whilst the silver salt yields an See also:oxygen ether, R•N: N•OCH3. These results point to the conclusion that the iso-diazo hydroxide is a tautomeric substance. The same oxygen ether is formed by the methylation of the silver salt of the normal diazo hydroxide; this points to the conclusion that the isomeric hydroxides, corresponding with the silver derivatives, have the same structural formulae, namely, R•N: N•OH. These oxygen ethers contain the grouping — N : N - , since they couple very readily with the phenols in alkaline solution to See also:form azo compounds (q.v.) (E. Bamberger, Ber., 1895, 28, p. 225) ; they are also explosive. By oxidizing potassium benzene iso-diazotate with alkaline potassium ferricyanide, E. Bamberger (Ber., 1894, 27, p. 914) obtained the diazoic acids, R•NH•NO2, substances which he had previously prepared by similarly oxidizing the diazonium salts, by dehydrating the nitrates of primary amines with acetic anhydride, and by the action of nitric anhydride on the primary amines. Concentrated acids convert them into the isomeric nitro-amines, the — NO2 group going into the nucleus in the ortho- or para- position to the amine nitrogen; this appears to indicate that the compounds are nitramines. They behave, however, as tautomeric substances, since their alkali salts on methylation give nitrogen ethers, whilst their silver salts yield oxygen ethers: potassium salt —~ R•N(CH3).NO2 nitramine. R•NH'NO2 silver salt R'N: N'O'OCH3 diazoate. Phenyl nitramine, C6H5NH NO2, is a colourless crystalline solid, which melts at 46° C. Sodium See also:amalgam in alkaline solution reduces it to phenylhydrazine. Constitution of the Diazo Compounds.—P. Griess (Ann., 1866, 137, p. 39) considered that thediazocompoundswereformedby the addition of complex groupings of the type C6H4N2— to the inorganic acids; whilst A. See also:Kekule (Zeit. f Chemie, 1866, 2, p. 308), on See also:account of their ready condensation to form azo compounds and their easy reduction to hydrazines, assumed that they were substances of the type R.N: N.Cl. The constitution of the diazonium group—N2•X, may be inferred from the following facts:—The group C6H5N2—behaves in many respects similarly to an alkali See also:metal, and even more so to the ammonium group, since it is capable of forming colourless neutral salts with See also:mineral acids, which in dilute aqueous solution are strongly ionized, but do not show any trace of hydrolytic See also:dissociation (A. Hantzsch, Ber.,1895, 28, p.1734). Again, the diazonium chlorides combine with platinic chloride to form difficultly soluble double See also:platinum salts, such as (C6H5N2CI)2•PtC14; similar See also:gold salts, C6H5N2Cl•AuC13, are known. Determinations of the See also:electrical conductivity of the diazonium chloride and nitrate also show that the diazonium radical is strictly comparable with other See also:quaternary ammonium ions. For these reasons, one must assume the existence of pentavalent nitrogen in the diazonium salts, in order to account for their basic properties. The constitution of the isomeric diazo hydroxides has given rise to much discussion. E. Bamberger (Ber.,1895, 28, pp. 444 et seq.) and C. W. Blomstrand (Journ. prakt.Chem., 1896, 53, pp. 169 et seq.) hold that the compounds are structurally different, the normal diazohydroxide being a diazonium derivative of the type R•N(N)•OH. The See also:recent See also:work of A. Hantzsch and his pupils seems to invalidate this view (Ber., 1894, 27, pp. 1702 et seq. ; see also A. Hantzsch, See also:Die Diazoverbindungen). According to Hantzsch the isomeric diazo hydroxides are structurally identical, and the See also:differences in behaviour are due to stereo-chemical relations, the See also:isomerism being comparable with that of the See also:oximes (q.v.). On such a See also:hypothesis, the relatively unstable normal diazo hydroxides would be the syn-compounds, since here the nitrogen atoms would be more easily eliminated, whilst the stable iso-diazo derivatives would be the See also:anti-compounds, thus: R.N R•N HON N•OH Normal hydroxide Iso hydroxide (Syn-compound) (Anti-compound) In support of this theory, Hantzsch has succeeded in isolating a See also:series of syn - and anti-diazo-cyanides and -sulphonates (Ber.,1895,28, p.666 ; 1900, 33, p. 2161 ; 1901, 34, p. 4166). By diazotizing para-chloraniline and adding a cold solution of potassium cyanide, a salt (melting at 29° C.) is obtained, which readily loses nitrogen, and forms parachlorbenzonitrile on the addition of copper powder. By dissolving this diazocyanide in alcohol and reprecipitating it by water, it is converted into the isomeric diazocyanide (melting at 105-106° C.), which does not yield para-chlorbenzonitrile when treated with copper powder. Similar results have been obtained by using diazotized para-anisidine, a syn- and an anti- compound being formed, as well as a third isomeric cyanide, obtained by evaporating para-methoxybenzenediazonium hydroxide in the presence of an excess of hydro-cyanic acid at See also:ordinary temperatures. This salt is a colourless crystalline substance of See also:composition CH 30.See also:C6H4.N2.CN•HCN•2H20, and has the properties of a metallic salt; it is very soluble in water and its solution is an electrolyte, whereas the solutions of the synand anti- compounds are not electrolytes. The See also:isolation of these compounds is a powerful See also:argument in favour of the Hantzsch hypothesis which requires the existence of these three different types, whilst the Bamberger-Blomstrand view only accounts for the forma- tion of two isomeric cyanides, namely, one of the normal diazonium type and one of the iso-diazocyanide type. Benzene diazonium hydroxide, although a strong See also:base, reacts with the alkaline hydroxides to form salts with the See also:evolution of See also:heat, and generally behaves as a weak acid. On mixing dilute solutions of the diazonium hydroxide and the alkali together, it is found that the molecular conductivity of the mixture is much less than the sum of the two electrical conductivities of the solutions separately, from which it follows that a portion of the ions See also:present have changed to the non-ionized See also:condition. Additional information and CommentsThere are no comments yet for this article.
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